1. Field of the Invention
The present invention relates to an organometallic complex. In particular, the present invention relates to an organometallic complex that can convert a triplet excited state into luminescence.
2. Description of the Related Art
An organic compound absorbs light, thereby producing an excited state. Through this excited state, various reactions (photochemical reactions) occur in some cases, or luminescence is generated in some cases. Therefore, the organic compounds have been variously applied.
One example of the photochemical reactions is a reaction of singlet oxygen with an unsaturated organic molecule (oxygen addition) (see Non-Patent Document 1, for example). Since the ground state of oxygen molecules is a triplet state, oxygen molecules do not produce a singlet state (singlet oxygen) when they are excited directly by light. In contrast, when oxygen molecules interact with triplet excited molecules other than oxygen, the oxygen molecules form singlet oxygen, resulting in an oxygen addition reaction. Here, the compound that forms the triplet excited molecules by light and enables formation of singlet oxygen is called a photosensitizer.
Thus, formation of singlet oxygen requires a photosensitizer that can form triplet excited molecules by light excitation. However, it is unlikely that a typical organic compound is converted to a triplet excited molecule because the ground state of the organic compound is typically a singlet state and photoexcitation to a triplet excited state is forbidden transition. For such a photosensitizer, a compound that can easily undergo intersystem crossing from the singlet excited state to the triplet excited state (or a compound that allows forbidden transition in which the compound is directly converted to a triplet excited state by photoexcitation) is thus needed. That is, such a compound can be used as a photosensitizer and regarded as being useful.
Furthermore, the above compound often emits phosphorescence. Phosphorescence refers to luminescence generated by transition between energies of different multiplicity. In an ordinary organic compound, phosphorescence refers to luminescence that is generated at the time of relax from a triplet excited state to a singlet ground state (in contrast, fluorescence refers to luminescence that is generated at the time of relax from a singlet excited state to a singlet ground state). Application fields of compounds that are capable of emitting phosphorescence, in other words, compounds that are capable of converting a triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element containing an organic compound as a light-emitting substance.
An example of a structure of such a light-emitting element is a simple structure in which a light-emitting layer containing an organic compound that is a light-emitting substance is merely provided between electrodes. Light-emitting elements having such a structure can achieve thinness, lightweight, high-speed response to signals, low-voltage DC drive, and the like. Therefore, attention has been directed to the light-emitting elements as next-generation flat panel display elements. Further, a display including this light-emitting element is superior in contrast, image quality, and wide viewing angle.
The light-emitting mechanism of the light-emitting elements in which organic compounds are used as the light-emitting substance is carrier injection. That is, by applying a voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes recombine to make the light-emitting substance excited, and light is emitted when the excited state relaxes to a ground state. As in the case of the photoexcitation, types of the excited state of organic compounds include a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
At room temperature, a compound that converts a singlet excited state into luminance (hereinafter, referred to as a fluorescent compound) emits light only from the singlet excited state (fluorescence), and does not emit light from the triplet excited state (phosphorescence). Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element formed using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, with a light-emitting element formed using a compound that converts a triplet excited state into luminance (hereinafter, referred to as a phosphorescent compound), the internal quantum efficiency can be improved to 100% in theory; namely, the emission efficiency can be 4 times as high as that of a light-emitting element formed using a fluorescent compound. Therefore, the light-emitting element formed using a phosphorescent compound has been actively developed in recent years in order to achieve a highly efficient light-emitting element (see Non-Patent Document 2, for example). An organometallic complex that contains iridium or the like as a central metal is particularly attracting attention as a phosphorescent compound because of its high phosphorescence quantum efficiency.